The proposed studies are derived from the hypothesis that stress- induced immunosuppression (SIIS) is mediated by discrete neuronal pathways in the CNS. The focus of the proposed studies is to determine the importance of certain brain regions in mediating stress-induced changes in splenic nerve-dependent immune responses in rats. The specific aims are to: (1) identify and neuronal circuits in the brain that innervate the spleen: (2) determine whether inhibition of discrete brain nuclei can eliminate SIIS in the spleen as assessed using an in vitro mitogen assay; and (3) determine whether inhibition of discrete brain nuclei can eliminate stress-induced suppression of the antibody response injection of sheep red blood cells. To determine which neuronal populations in the brain innervate the splenic sympathetic preganglionic neurons, a transneuronal retrograde transport mapping study will be performed; pseudorabies virus will be injected into the spleen, and the subsequent passage of this neurotropic virus through synaptically-linked neural circuits will be examined using light microscopy immunohistochemical techniques. To examine the importance of specific brain regions in SIIS, a neural-circuit based approach will be used to determine whether acute inhibition of neuronal function in discrete brain sites during footshock stress alters SIIS in the spleen. This will be assessed using both an in vitro non-specific mitogen assay (Aim 2), and an antigen-specific lymphocyte response assay (aim 3). Parallel studies examining Fos expression using immunohistochemical methods will be performed to evaluate the effectiveness of our methods to inhibit neuronal function. In addition, these studies will provide data on the importance of specific brain sites in stress- induced Fos expression in neuronal populations throughout the brain. Results from the proposed studies will help delineate the neural circuits that mediate SIIS, and provide the framework for examining rational therapeutic approaches to reduce the impact of stress-related effects on immune system function and disease susceptibility.